The present invention relates to radiation measurement, and, in particular, to radiation measurement of dose enhancement in radiation facilities.
It has become increasingly clear in recent years that the conventional dosimetry methods used in testing electronic systems can lead to an underestimate of the dose received by irradiated piece parts. In typical cobalt-60 test facilities the magnitude of the errors can easily exceed a factor of two and in some cases a factor of five. This problem, which is now commonly referred to as dose enhancement, arises due to a fundamental assumption implicit in present day gamma and X-ray dosimetry methodology. The assumption is that the dimensions of the target material are large relative to the range of the energetic Compton electrons and photoelectrons produced by the incident high energy gamma photons. Under this assumption transport of the secondary electrons away from the point where they are produced is neglected. This greatly simplifies the interpretation of dosimetry measurements and calculations.
The dimensions of modern microelectronic devices are such that the basic assumption underlying common dosimetry techniques, i.e., zero secondary electron transport, is almost never justified. The result is the possibility of the substantial errors previously mentioned.
To compound the problem it turns out that the magnitude of the dose enhancement is very sensitive to the gamma spectrum at the point of interest. This spectrum is very different from that emitted by the source due to Compton scattering within the test facility. The scattered spectrum is also quite variable from one test facility to another. Large variations are even possible in the same facility due to changes in supporting structures or shielding materials in the immediate vicinity of the object under test.
The direct calculation of the enhancement is difficult for two reasons: because of scatter it requires a determination of the gamma photon spectrum at the point of interest (a photon transport problem), followed by a second calculation of the energy deposited by the photon induced secondary electrons in the target structure (an electron transport problem). For the complex three dimensional geometries commonly encountered, Monte Carlo computations are generally required in both cases. Fortunately, improvements in transport codes in recent years have enabled calculations to be made for even the complex multilayered structures typically encountered in device testing. There are additional problems, however, in the routine application of the computational approach.
Accurate input data required to perform the calculations is likely to be unavailable. The photon transport part of the calculation in a given test facility requires data on the exact positions, shapes and compositions of the structures surrounding the test object. The electron transport part of the calculation requires information on the microscopic structural details of the irradiated device or component. Inaccurate information concerning either or both of these areas can render the transport calculations inaccurate.